A clamp meter senses the magnetic field around a conductor, with a current transformer core for AC or a Hall-effect sensor for AC and DC, so current can be read without disconnecting anything. Most also measure voltage, resistance and continuity through test leads. Clamping around a whole cable reads near zero because go and return currents cancel, a trick used deliberately with leakage clamps to find earth leakage in milliamps.
Leakage clamp meter round the whole cable reads 12 milliamps, so something on that circuit is leaking to earth.
Australians and Brits often say tong tester; North Americans say amp clamp or clamp-on.
Also heard as
- clamp-on ammeter
- amp clamp
- tong tester
Related terms
True RMS
Describes a meter or reading that computes the real RMS of any waveform, not one scaled from a sine-wave average.
RMS
Root mean square: the effective value of an alternating quantity, giving the same heating as that DC value.
Nuisance trip
Protective device opening with no real fault present, usually from bad settings, leakage or transients.